How the new navigation pivot is determined depends a bit on the kind of brush:
* Brushes that deform or remove curves use the 3d-brush position at the start of
the brush.
* Brushes that add new curves set the pivot to the bounding box center of the
new curves.
Finding a good pivot point is not super trivial for curves, but the existing 3d
brush functionality seems to work well. This also has the benefit that almost no
additional computation is needed when the user is using the spherical brush
mode. However, if the projected mode is used, and orbit-around-selection is on,
then we have to compute the spherical brush center now anyway.
Pull Request: https://projects.blender.org/blender/blender/pulls/131907
97fe1387ae